In the process of filtration, cartridges are used to separate the solid particles from liquid. Filter cartridges are generally of hollow cylindrical shape. The surface of the cylinder usually contains fine openings. When liquid passes from outside of the cylinder to the inside, these openings will trap and retain all solid particles larger than their own size. Such cartridges are installed inside a vessel and the liquid will be passed through them under pressure. The cylindrical surfaces can be made of metal, plastic or fiber. The size of the openings will determine the particle size to be trapped and the rate of flow of liquid. Once the particles start filling up the openings, this restriction will cut down the flow. When the particles fill up all the openings, the cartridge is then referred to as being "blinded". Once the cartridge is blinded, it is necessary to remove it. The majority of blinded cartridges cannot be reused and must be discarded. In some systems the cartridge may be taken out and mechanically cleaned or chemically cleaned and if they are still functional, they may be reused. This results in expensive down time and/or wasted cartridges and adds to the cost of the filtration process.
In some prior art filtration systems, strainers in the form of a helical strip of metal or other suitable material have been used. One example is shown in U.S. Pat. No. 2,569,748. In that patent the dirty liquid is flowing from inside the helix to outside the helix. The coils of the helix are spaced apart a predetermined distance to prevent the passage of predetermined size solids outwardly between the coils. The strainer apparatus in that patent is of the backwash type in which a counter-flow of liquid is employed to remove solids form the spaces between the coils of the helix to increase the spacing between the coils. Other helical coil filters are shown in U.S. Pat. Nos. 2,517,825, 3,179,116 and 4,804,481.